The present invention relates to triggering devices for electronic switches such as thyristers, triacs and MOS power switches.
Triggering devices for operating thyristers and triacs are known which have a structure so as to galvanically decouple the control electronics from the controlled device. One such known device employs a transformer with two windings which are not galvanically connected to one another and which are disposed between a thyrister to be controlled and the control electronics. Another known triggering device employs an opto-electronic coupling element for the same purpose.
The use of a transformer as the decoupling structure as described above requires a capacitor be connected in parallel to that secondary winding of the transformer which is connected to the trigger electrode of the thyrister so that noise signals proceeding from the output side or from the load side of the thyrister do not trigger the thyrister via the anode-to-grid capacitance of the thyrister. If an opto-electronic coupling element is employed, an auxiliary voltage supply must also be provided on the secondary side. The use of a capacitor in the transformer structure or an auxiliary voltage supply in the optoelectronic structure represents an additional circuit outlay and expense associated with those types of devices.
A circuit for galvanically separated drive of a SIPMOS power switch is known from the publication "Siemens Components", 18, No. 4 (1980), pages 187-188 wherein a transformer having three windings is employed for galvanically separate transmission of the drive signal. An exemplary circuit for such a drive with a transformer is shown in FIG. 1 of the publication. A SIPMOS power switch or power circuit breaker is a semiconductor component which is essentially a field effect transistor manufactured according to known MOS technology such that large currents can be switched by the component. Such power switches may be employed, for example, for switching the operating current of a motor. Further details of such MOS power switches can also be found in "Siemens Forschungsund Entwicklungsberichte," Vol. 9, No. 4 (1980) at pages 181-189.
Galvanically decoupled drive of such MOS power switches is desirable in almost every application where protection is required as a result of the presence of high voltages. A MOS power switch can be reliably switched on and off with six volts, although the voltage to be switched by the power switch can amount to several hundred volts such as, for example, bus line voltage.
A micro-relay or an opto-electronic coupler consisting of a photodiode at the input side and a phototransistor at the output side may be employed for galvanically decoupled drive of a MOS power switch, however, the use of such devices results in additional outlays such as, for example, auxiliary circuits, particularly for current limitation and further necessitates the presence of an additional voltage source on the load side.
A trigger device with a piezo-coupler which does not require such additional circuit outlay is claimed in the co-pending application of Valentin Magori, filed on Apr. 8, 1981 and assigned U.S. Ser. No. 252,080.